1. Field of the Invention
The present invention relates to a reception synchronization apparatus for use in a mobile communication system or the like and to a modulating apparatus using such a reception synchronization apparatus. More particularly, the present invention relates to an improved reception synchronization circuit for receiving data modulated by means of an OFDM (Orthogonal Frequency Division Multiplexing) technique.
2. Description of the Related Art
In recent years, mobile communication using a portable telephone or the like has become increasingly popular. Mobile communication is used to transmit not only information with a small data size such as voice data but also information with a large data size.
In a mobile communication system, as shown in FIG. 9, a plurality of base stations BS are distributed in a ground plane so that a mobile station MS can communicate with a base station BS located near the mobile station MS.
Herein, an area within which a base station can communicate with a mobile station is referred to as a cell.
In such a mobile communication system, in order to avoid cross talk, each cell uses a frequency different from those used in adjacent cells.
However, the same frequency channel can be used in a more distant cell outside the adjacent cells without encountering a significant problem, because, for a mobile station MS being in a cell, the strength of a signal received from a base station BS of that cell is greater than that of an interfering signal coming from a distant cell.
If the distance among cells in which the same frequency channel is used is set to be very large, a large number of different frequency channels are necessary, and thus the spectrum efficiency becomes low. That is, there is a trade-off between the interference due to usage of same frequency channel and the spectrum efficiency.
Thus, it is important to design a communication system such that the system has high resistance against interference thereby achieving an improvement in the spectrum efficiency.
More specifically, in a communication system using the OFDM modulation technique, a plurality of time slots TSLT each including an effective symbol period TSBL and a guard period TGD are combined into a frame FRM, as shown in FIG. 10, and transmitted from a base station BS. In the example shown in FIG. 10, each frame FRM includes three time slots.
Base stations BS are synchronized in terms of transmission so that frames are transmitted with the same timing.
The purpose of a guard period TGD added to each effective symbol period TSBL is to suppress intersymbol interference due to multipath transmission or fading.
Each time slot including a guard period TGD is produced, as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 7-99486, by connecting the same signal as a predetermined length of head or tail end part of a signal in an effective symbol period to an opposite end of that effective symbol period or by connecting the same signals as predetermined length of both head and tail end parts of a signal in an effective symbol period to opposite ends of the effective symbol period. More specifically, the same signal as a signal at a tail end part of an effective period is connected to the head end of the effective symbol period, or the same signal as a signal at a head end part of an effective period is connected to the tail end of the effective symbol period, or otherwise, the same signals as signals at head and tail end parts of an effective period are respectively connected to the tail and head ends of the effective symbol period.
In a receiving system of a mobile station that receives such an OFDM signal, as shown in FIG. 11, the correlation is determined between the received OFDM signal and a signal obtained by delaying the OFDM signal by a time equal to one effective symbol period. The start positions of respective effective symbol periods are then determined from peak positions of detected in the correlation. That is, it is possible to determine the location of a guard period in each time slot.
The detection of the start position of an effective symbol period allows an OFDM demodulator to perform an FFT (Fast Fourier Transform) operation.
An example of such an OFDM demodulator is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 8-107431.
In the OFDM demodulator disclosed in the Japanese Unexamined Patent Application Publication No. 8-107431 cited above, the correlation between a received OFDM signal and a signal obtained by delaying the received OFDM signal by an effective symbol period, and the resultant correlation signal is subjected to an interval integration.
In the above process, the interval integration is performed, as shown in FIG. 12, for intervals created by dividing the correlation signal into segments that is, intervals, each having a length equal to the time slot period.
That is, the cumulative sum of the correlation signal is determined by repeatedly adding the correlation signal in the respective intervals. In the resultant signal, peaks appear at particular positions within the time slot period as shown in FIG. 12(E). In parts where there is no correlation, the values are averaged as the interval integration advances.
As described above, the interval integration makes it possible to clearly distinguish a correlated period from an uncorrelated period, and the detection of a peak makes it possible to achieve synchronization in a more reliable fashion.
In the communication system using an OFDM signal added with a guard period, as described above, although intersymbol interference due to multipath transmission or fading can be suppressed, there is still a possibility that a mobile station encounters interference when receiving the OFDM signal added with the guard period in some situations.
A mobile station receives a signal in such a manner as described below.
In addition to a desired wave DSW, a mobile station also receives an interfering wave IFW via the same channel. In most cases, the interfering wave IFW does not cause a problem, because the reception signal strength of the desired wave DSW is much greater than that of the interfering wave IFW.
However, fading occurs as a mobile station moves, and thus the reception signal strength of the desired wave DSW and that of the interfering wave IFW frequently vary.
In general, there is no correlation between fading of a desired wave DSW and that of an interfering wave IFW. That is, the desired wave DSW and the interfering wave IFW fluctuate independently of each other. This means that the reception signal strength of the interfering wave IFW can become high when that of the desired wave DSW becomes low. In such a case, there is a possibility that interference makes it impossible to receive the desired wave DSW.
In general, an interfering wave IFW arrives at a mobile station slightly later than a desired wave DSW, because the interfering wave IFW is transmitted from a base station at a more distant location while the desired wave DSW is transmitted from a base station at a closer location.
Referring to an example shown in FIG. 10, a possible reception of an interfering wave IFW is discussed below for a case in which a fluctuation in the reception signal strength due to fading causes a signal transmitted from a distant base station using the same channel to be received as an interfering wave IFW. It is assumed herein that only one frame is received as the interfering wave IFW as shown in FIG. 10(B).
In contrast, in the case of a desired wave DSW, frames are successively received as shown in FIG. 10(A).
Because the interfering wave IFW arrives slightly later than the desired wave DSW as shown in FIG. 10(B), the interfering wave IFW interferes with two frames, denoted by (i) and (ii) in FIG. 10, of the desired wave DSW.
One possible technique to improve the resistance of a communication system against interference caused by use of the same channel is to use a frame guard FGD as shown in FIG. 13.
In the example shown in FIG. 13, a frame FRM includes a frame guard period TFGD at the tail end thereof.
Unlike a guard period TGD disposed before or after an effective symbol period SBL, the frame guard period TFGD includes no signal.
Therefore, even if a frame guard disposed at the tail end of a frame of an interfering wave IFW overlaps with a head end part of a desired wave DSW, no interference occurs.
Thus, in the example shown in FIG. 13, although the interference causes the first frame, denoted by (iii) in FIG. 13, of the desired wave DSW to become impossible to be received, the second frame denoted by (iv) can be received without encountering interference.
However, in the convention OFDM demodulating apparatus described above, when the OFDM signal including frame guards is subjected to cumulative addition on a slot-by-slot basis in order to detect a synchronization position, the frame guards present in the middle make it impossible for a correlation peak to appear at a fixed position as shown in FIG. 14, and thus it is impossible to precisely detect the synchronization position.
In view of the above, it is an object of the present invention to provide a reception synchronization apparatus capable of achieving precise synchronization of an OFDM signal including an additional frame guard. It is another object of the present invention to provide a demodulating apparatus using such a reception synchronization apparatus.